Fix APPEND/NCONC type derivation properly this time.

Fix APPEND/NCONC type derivation properly this time.

Use type-intersection for checking types, it's more robust than what
was there before.

And a slight improvement. When argument in the middle can't be a NIL,
then the end result is guaranteed to be a CONS. Previously, the
assumption was if the type is a CONS, but that doesn't work with types
like (or cons vector).